Resin adhesives are used to establish bonded joints between the mating surfaces of adhered structures. Typical of such structures are the metal or composite laminate structures used in various applications such as airframe or surface skin components of aircraft or space vehicles. Such structures may be formed through the use of adhesives to bond one laminate ply to another. Two metal surfaces such as aluminum or tungsten surfaces may be bonded together or a composite material, e.g. graphite, a carbon-carbon composite or reinforced plastic or refractory composites may be bonded to another composite or metal surface. For example, one form of composite panel can comprise alternate layers of a graphite composite and a titanium alloy.
In bonding laminate plies or other mating surfaces to one another, it is a conventional practice to add thermosetting adhesive resins which are applied to one or both adhered substrates after which the substrate surfaces are brought together, usually under pressure and with heating to cure the adhesive so that it provides a rigid joint. Numerous thermosetting resins are useful in adhesive applications. The resin of choice in a particular application will depend upon many factors, such as the character of the substrate surfaces to be joined, the environment in which the structure is to be used, e.g. considering such factors as temperature, moisture conditions and the like, and the strength desired at the bonded joint.
Various thermosetting resins and their uses in bonding different materials are disclosed in Thompson, R., "Assembly of Fabricated Parts, Adhesive Bonding", Modern Plastics Encyclopedia, October, 1986, Vol. 63, No. 10A, pp 350-354, McGraw-Hill Publications Company, New York, N.Y. Of the adhesive resins typically used in aircraft or space structure applications; rigid epoxy and polyurethane type resins usually are preferred. Others which are also used in such applications include polyimide resins, polyamide resins, usually where atmospheric moisture levels are controlled in the use environment, and polysulfide resins, where an usually high degree of flexibility is required.
Oftentimes primer coatings are applied to the substrate surfaces, and in some cases, prior to applying the primer coating, a substrate surface is subjected to a surface treatment to increase the adherence between the primer coating and the substrate surface. Such primer coatings may take the form of resinous adhesive materials which may be the same or different than the adhesive layer itself. Most primers involve epoxy resins although other resins suitable for primer applications include polysulfides, silicons and bismaleimide polymers. The primer, in addition to providing a good bondable surface for the adhesive layer, also serves as a corrosion protective coating for the substrate material in the bond line area. The primer layer may be required to endure for days, weeks or even months. One widely used protocol for the fabrication of aluminum alloy faced sandwich structure is disclosed in Aerospace Materials Specification AMS-3911A, Society of Automotive Engineers, revised Apr. 1, 1983. As described there, after treating the surface by degreasing followed by acid etching, primer is applied by spraying the surface to provide a coating 0.0001-0.0007 inch. After spraying, the primer coating is initially dried for at least 30 minutes at about 75.degree. F. and thereafter cured for 75-90 minutes at a temperature of 230.degree.-250.degree. F. U.S. Pat. No. 3,663,354 to Veno et al. discloses a primer suitable for use on metal adhered substrate surfaces where the adhesive resin is in the form of a linear polyimide cement. The primer is described as a curable precondensate of a resol-type phenolic phenol formaldehyde resin and an epoxy resin. The primer may be applied to the substrate surface in a molten state or sprayed in a solvent solution with the solvent then being evaporated. Before application of the linear polyimide adhesive, the primer coating is cured at 170.degree.-230.degree. C. for 3-30 minutes to provide a coating having a thickness of about 1-20 microns.
Another process involving the use of thermosetting resins to join metallic components together employing a primer coating and a polyimide adhesive is disclosed in U.S. Pat. No. 3,936,342 to Matsubara et al. Here, a primer comprising a mixture of a bisphenol A type epoxy resin and a phenolic resin is employed as a primer. In applying the primer to metallic surfaces, the resin formulation may be dissolved in ketone solvents such as acetone or methylethyl ketone, aromatic compounds such as toluene or xylene, glycols, esters or alcohols and applied to the substrate surface. The solvent is removed from the coated resin layer by drying and the resin is cured to provide a film having a thickness of about 1-10 microns, particularly 2-6 microns. Drying may be accomplished in an air stream at 60.degree. C., and the primer coating then cured at a temperature of 180.degree.-300.degree. C. for 30 seconds to 15 minutes. The polyimide resin is applied at a temperature of 200.degree.-300.degree. C. to provide an adhesive layer of thickness of about 10- 200 microns. The structure is then cooled with the polyamide resin providing an adhesive between the primer resin layers.
Another procedure employing resin priming of metallic surfaces is disclosed in U.S. Pat. No. 4,035,436 to Matsubara et al. Here, the primer coating is provided by an epoxy-phenol resin, a phenol resin, an epoxy urea resin, an epoxy ester resin or an epoxy amino resin. Similarly, as in the case of the aforementioned Matsubara '342 patent, the resin coatings are cured by heating, e.g. at 160.degree.-220.degree. C. for 5-10 minutes, and a polyamide adhesive is used to bond the substrate surfaces together. Disclosed in this reference are nylon-based polymers which may be modified by the addition of other polymers such as fatty acid polyamides and polyethylenes.
U S. Pat. No. 4,169,006 to Matsubara et al. discloses a primer coating provided by the reaction product of a polyester resin obtained as the reaction product of a polycarboxylic acid component and a polyhydric alcohol component and a bisphenol A type epoxy resin. As in the previous Matsubara patents, the resin coatings are baked at high temperatures, e.g. 160.degree.-185.degree. C. for 10 minutes, in order to effect a curing of the resin coating.
U.S Pat. No. 4,022,649 to Nakagome et al. discloses a process for producing metal laminate structures of high thermal resistance in which one resin coating is dried to provide a prelaminate coating flow residual volatile content and another coating is of a higher volatile matter content. A resin material characterized as a polymer containing heterocyclic rings is supplied to a substrate surface and dried to provide a film having a thickness of 20-300 microns with a residual volatile content ranging from zero up to 3% by weight. This film is heated at a temperature of about 250.degree.-350.degree. C. for up to 2 hours. In a specific example, the initially applied polymer was heated at 100.degree. C., 180.degree. C. and 260.degree. C. for 30 minutes to provide a prelaminate layer having a residual volatile content of 0.8% and a thickness of 50 microns. A second solution of a polymer, also characterized as a polymer containing heterocyclic rings, is applied to the other substrate surface in a thinner film, for example, 2-20 microns and dried to provide a film having a higher volatile matter content of 4-20% by weight. The two mating surfaces are brought together at a high temperature, e.g. 270.degree. C. for 30 minutes.